This application is based on application No. 2001-188739 filed in Japan, the content of which is hereby incorporated by reference.
The present invention relates to an objective lens system for optical pickups, for example, to an objective lens system for optical pickups having a large numerical aperture (NA) and used as the objective lens system in optical pickups mounted in optical information recording apparatuses, magneto-optic recording apparatuses and the like.
Conventionally known optical information recording apparatuses use, as the objective lens systems for optical pickups, objective lens systems performing at least one of reading and writing of information from and onto information recording media (optical discs, etc.). Objective lens systems for optical pickups have various types. For example, as objective lens systems for optical pickups for blue lasers, two-element objective lens systems having positive refractive power are proposed in Japanese Laid-Open Patent Applications Nos. 2000-75107, 2000-206404 and 2000-180717.
The objective lens systems for optical pickups proposed in Japanese Laid-Open Patent Applications Nos. 2000-75107 and 2000-206404, the NA is increased by using a substantially hemisphere plano-convex second lens element (information recording medium side lens element). However, since the first lens element (light source side lens element) is bi-convex, it is difficult to secure a back focal length of the objective lens system for optical pickups, so that the thickness of the disc substrate cannot be increased. In the objective lens system for optical pickups proposed in Japanese Laid-Open Patent Application No. 2000-180717, the second lens element is a meniscus lens convex to the light source side. For this reason, it is difficult to secure an interval between the lens periphery and the disc substrate even if the back focal length is increased. Thus, this objective lens system is undesirable with respect to the working distance.
An object of the present invention is to provide an improved objective lens system for optical pickups.
Another object of the present invention is to provide an objective lens system for optical pickups being high is performance and small in size while having a large NA and securing a working distance.
The above-mentioned objects are achieved by an objective lens system for optical pickups performing at least one of reading and writing of information by condensing a luminous flux from a light source on an information recording medium, said objective lens system comprising the following two lens elements from a light source side: a first lens element having a first surface convex to the light source side and a second surface convex to the light source side; and a second lens element having a third surface convex to the light source side and a plane fourth surface, wherein the first surface is an aspherical surface.